Analysis of Zinc

For analytical lab, there are certain procedures that are not given in the manual - they must be looked up. References are usually, but not always, given.

When there are references, the procedure is trivial. Go to the library, borrow the book from reserves, find the relevant pages, photocopy. Expensive (as school photocopiers always are), but easy. If the reference is an article, life is even easier, due to internet and a wealth of journal subscriptions (yay for relatively affluent schools!).

Life is a bit weirder when there are no references given.

As an example, one of my experiments involve analyzing the elements in a brass sample. Damn, I miss the ICP at the workplace (life would be so much easier if I have that). Even an AA would be nice. Being a second-year lab, we go the old-school way: tin by precipitation, copper and lead by electrodeposition, zinc by a method of choice that the student looks up (and checked over by the lab instructor for approval).

Of course, there are restrictions. No million-dollar equipment (which rules out the lovely ICP* *sad*). Nothing too exotic or toxic (no cyanides, hydrogen sulfide, rhodium, etc.). No significant interferences from residual compounds from previous analyses. And it would be nice if it isn't a fussy, lengthy procedure that requires waiting for equipment, because time is at a premium (as always).

The problem is, there are lots of ways to analyze for zinc in a sample. Most of which do not fulfill the above criteria, and even if they do, will have quite a few more disadvantages than advantages. For example, a canonical method would be titration with EDTA. But copper masks the indicator that the lab has, and the appropriate demasking agents are toxic. No go. Anyway, I think I've mentioned how much I hate EDTA titrations before.

Because I too am old-fashioned, I looked in books, like Vogel's Quantitative Chemical Analysis. Older books are generally a wonderful resource for classical techniques, as many may know.

Well, that much is true. However. There's the zinc ammonium phosphate/pyrophosphate method (the one most people, including me, will use). The zinc oxalate method, but what about copper and lead oxalates? There's the zinc-[illegal drug precursor] method. There's an iodimetric titration that intentionally generates hydrogen sulfide. There's a modified Volhard titration, where the zinc is precipitated as ZnHg(SCN)₄, dissolved, and back-titrated. There's more electrodeposition, which I'd rather not do because of equipment requirements. And there's a whole lot more other reactions out there, which I didn't bother to photocopy.

I would especially love to see my instructor react if I submitted the reaction where there's a ferrocyanide titration...with uranyl nitrate as an indicator. Please? Pretty please?

Incidentally, in a previous post I've mentioned that for analytical lab, I need to pick two specific techniques out of a list. I think GC versus LC will depend on which has a shorter waiting list, while the other experiment, well, if they are going to give tricks for how to develop the methods for an AA/ICP or autotitrator, then I'll take that (because they are quite common in my limited industrial experience), and voltammetry otherwise. (Ion-selective electrodes just seem so...meh. They are just modified pH meters, in a sense.) See how that goes...

*Strictly speaking, a nice ICP-OES might not cost a million dollars. It does cost more than a nice house in a large city, though, and a house costs alot in Canada these days.